lecture 15 notes - Today in Astronomy 111: giant planets...

27 October 2011 Astronomy 111, Fall 2011 1
Today in Astronomy 111: giant planets
and planetary atmospheres
 The other giant planets
• Vitals of Saturn,
Uranus and Neptune
• Gas giants and ice
giants
 Vertical density and
pressure structure of
atmospheres
All four giant planets , on the same scale
(Voyager images, JPL/NASA).

27 October 2011 Astronomy 111, Fall 2011 2
Saturn’s vital
statistics
29
9
-3
2
14
Mass 5.6846 10 gm (95.2 )
Equatorial radius 6.0268 10 cm (9.45 )
Average density 0.687 gm cm
Moment of inertia 0.210
Bond albedo 0.342
1.43353 10 cm
Orbital semimajor axis
(9.582 AU)
Orbital eccentrici
M
R
MR
⊕
⊕
×
×
×
ty 0.0565
Obliquity 26.73
Sidereal
29.457 years
revolution period
Sidereal
10.656 hours
rotation period
Moons 62 and counting
Rings 7 major ones
°
Saturn, from
Cassini (JPL/NASA)

27 October 2011 Astronomy 111, Fall 2011 3
Visits to Saturn
We have visited Saturn four
times, including the visit
currently in progress:
 Pioneer 11 (1979)
 Voyager 1 (1980)
 Voyager 2 (1981)
 Cassini (2004-)
The Voyager family portrait of
Saturn and some of its larger moons
(JPL/NASA)

27 October 2011 Astronomy 111, Fall 2011 4
Saturn: structure and composition
Like Jupiter, Saturn is a gas giant.
 Has the lowest average density of the planets, and a very
low moment of inertia for its mass.
 Spins almost as fast as Jupiter, and its visible surface is
even more distorted by its rotation than Jupiter (polar
diameter 10% smaller than equatorial diameter), owing to
lower density and larger rocky core.
 Definitely has a rocky core,
 Elements heavier than H even more abundant than in
Jupiter; e.g. . Visible constituents:
96.3% H
2
, 3.25% He, 0.45% CH
4
, 0.013% NH
3
, 0.011% HD,
0.0007% C
2
H
6
, 0.0004% H
2
O.
 T = 95 K at the cloudtops. Compare to 83 K expected from
heating by sunlight.
~ 12 . M
⊕
C H 10(C H) ≈


27 October 2011 Astronomy 111, Fall 2011 5
Saturn: structure and composition
 Thus Saturn emits 2.5 times as much power as it receives
in sunlight, similar to Jupiter.
• Related to major abundance difference from Jupiter?
There’s much less helium in Saturn’s upper
atmosphere, leading to suggestions of formation and
precipitation of liquid helium droplets ( helium rain )
that gradually raises the density of the interior (thus
reducing potential energy, increasing heat).